PROJECT SUMMARY Exertional rhabdomyolysis (ER) is a painful condition characterized by rapid-onset muscle cell necrosis and subsequent leakage of intracellular contents into the systemic circulation in response to exercise. Genetic fac- tors are especially important in pediatric patients, military recruits, and athletes with recurrent episodes of rhabdomyolysis; however, approximately half of these cases fail to receive a definitive diagnosis, leading to a lack of targeted therapy. A deeper understanding of the possible etiologies of recurrent ER (RER) will provide useful insight into its pathophysiology, which could eventually translate to the refinement of treatment and pre- vention strategies. The domestic horse is uniquely suited to serving as a naturally occurring model for exer- tional muscle disease. RER has a major impact on health in Thoroughbred (TB) and Standardbred (STB) horses, with many of the same clinical signs and risk factors as in human patients. There is strong evidence for a genetic component conferring RER susceptibility in both breeds; however, the specific genetic variants con- tributing to this susceptibility are currently unknown. Results from in vitro contracture tests are suggestive of a defect in muscle contractility in RER-susceptible horses that is physiologically and biochemically distinct from known forms of malignant hyperthermia. Therefore, the hypothesis driving this project is that RER in TB and STB horses is caused by functional genetic variants affecting the regulation of skeletal muscle contractility. Preliminary genome-wide association studies (GWAS) have identified seven chromosomal loci significantly associated with RER. The goal of this project is to expand on these preliminary analyses and identify the ge- netic variants underlying RER skeletal muscle dysfunction through the following specific aims: 1, Identification of candidate genes for RER susceptibility; and 2, Identification of putative causal variants for RER. In aim 1 candidate genes for RER susceptibility will be determined via GWAS and prioritized using computational methods that capitalize on knowledge of rhabdomyolysis pathophysiology for humans and other species. In aim 2, analysis of whole genome sequence will be employed to identify potentially functional alleles within these candidate genes that occur at higher frequency in RER cases than controls. High-priority variants will be used to create a custom genotyping assay to genotype our entire GWAS cohort as well as independent valida- tion cohorts to identify the true functional variants. The proposed work is expected to identify genes of moder- ate to major effect contributing to RER susceptibility and will pave the way for future studies focused on as- sessment of the physiologic function of causative variants. Results from this project are expected to advance overall understanding of altered muscle contractility and RER pathophysiology and therefore lead to the even- tual improvement of prevention and treatment regimes.